Measuring instrument

ABSTRACT

A measuring instrument that includes a tube  21  having a female thread  211  and a spindle  3  having a lead screw  31  screwing with the female thread  211  and capable of being advanced and retracted in an axial direction along with rotation around the axial center, and measures dimensions etc. of a workpiece according to displacement in an axial direction of the spindle  3  based on a rotation amount of the spindle  3.  According to this measuring instrument, a pitch P of the lead screw  31  is twice as large or more than the difference between an external diameter R and a core diameter r thereof, and the difference between the external diameter R and the core diameter r is one-fifth or less of the external diameter R. Because of the lead screw  31  with the large pitch, the spindle  3  can be moved at high speed, thus enhancing operational performance of the measuring instrument.

TECHNICAL FIELD

The present invention relates to a measuring instrument that measuresdimensions etc. of a workpiece by advancing and retracting a spindlealong with its screwing rotation, for instance, the present inventionrelates to a measuring instrument as typified by a micrometer or amicrometer head.

BACKGROUND ART

Conventionally, there has been known a measuring instrument, forinstance, the one typified by a micrometer, a micrometer head or thelike, that has a main body provided with a female thread and a spindleprovided with a male screw, and measures dimensions etc. of a workpieceby advancing and retracting the spindle along with its screwingrotation. An example of the measuring instrument is disclosed in, forinstance, Reference 1: JP S49-80260U, Reference 2: JP S54-130152A.

According to such a measuring instrument, displacement per rotation ofthe spindle is defined by a screw pitch of the male screw formed on thespindle.

The screw pitch of the male screw formed on the conventional spindle istypically 0.5 mm or 0.635 mm.

However, since the screw pitch of the male screw formed on the spindleis 0.5 mm or 0.635 mm, the spindle must be rotated for many times as anobject to be measured is changed because the displacement per rotationof the spindle is small, thus raising a problem on its operationalperformance.

Here, it is conceivable that a multiple thread screw is applied to themale screw of the spindle for increasing the displacement per rotationof the spindle. For example, a triple thread screw may be employed totriple the displacement per rotation. However, in order to machine themultiple thread screw precisely, a plurality of outside helixes must beformed with accurate phase difference. In the case of the triple threadscrew, although three outside helixes for a screw need to be formed withthe phase difference by 120 degrees, it is difficult to machine such amultiple thread screw with that phase difference maintained precisely,and machining error may cause measurement error. Additionally, it isalso difficult to accurately form many outside helixes with the equalpitch, thereby increasing the machining cost thereof as the number ofthe outside helixes increasing.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a measuring instrumentrealizing high measurement accuracy and high-speed movement of a spindleby solving the above-described problems. Therefore, the presentinvention employs the following configuration.

A measuring instrument according to an aspect of the present inventionincludes: a main body having a female thread; and a spindle having alead screw screwed with the female thread and capable of being advancedand retracted in an axial direction along with rotation around the axialcenter, in which a pitch of the lead screw is twice as large or morethan the difference between an external diameter and a core diameter ofthe lead screw, and the difference between the external diameter and thecore diameter is one-fifth or less of the external diameter.

With such a configuration, when the spindle is rotated, the spindle isadvanced and retracted along with screwing rotation of the spindleagainst the main body. Counting a rotation amount of the spindle at thistime, the displacement of the spindle can be figured out according tothe movement pitch per rotation of the spindle, thus obtaining themeasurement value.

Since the lead screw has the large pitch twice as large or more than thedifference between the external diameter and the core diameter, themovement pitch per rotation of the spindle can be enlarged. Accordingly,the spindle can be moved at high speed, and the operational performanceof the measuring instrument can be enhanced. While the spindle needs tobe displaced according to an object to be measured as the object ischanged, the spindle can be advanced and retracted at high speed withthe small rotation number if the lead screw has the large pitch, thusreducing effort and time spent on the measurement.

When the thread groove depth is increased for enlarging the screw pitch,the machining allowance for cutting the spindle will be increased andthe strength thereof will be deteriorated. This may cause a risk ofdeteriorating the measurement accuracy caused by deflection or the likeof the spindle. However, the strength of the spindle can be ensuredenough by restricting the difference between the external diameter andthe core diameter of the lead screw to be one-fifth or less of theexternal diameter, thus the measurement accuracy can be highlymaintained.

Preferably, adjacent thread grooves of the lead screw may be formed tohave predetermined intervals in a direction along a screw axis line, andan intermediate portion of thread grooves may be defined between theadjacent thread grooves as a straight line along the screw axis line ona cross-section along the screw axis line.

With this arrangement, the screw pitch is enlarged for the predeterminedintervals when the predetermined intervals are provided between theadjacent thread grooves. Accordingly, the lead screw having the largepitch can be provided without the thread grooves cut deeply.

Preferably, the female thread may have screw threads of the same pitchas that of the thread grooves, as seen in a direction along the screwaxis line of the female thread, the adjacent screw threads being formedwith predetermined intervals, and an intermediate portion of threadsbeing defined between the adjacent screw threads as a straight linealong the screw axis line on a cross-section along the screw axis line.

With this arrangement, since the female thread has screw threads only atthe part where the female thread fits to the thread grooves of the leadscrew but does not have screw root threads at the intermediate portionof threads, the machining allowance will not be increased even in thecase of the female thread screwing with the lead screw with the largepitch. Accordingly, since the main body needs not be cut deeply, thestrength of the main body can be ensured.

Preferably, in the present invention, the measuring instrument mayinclude: the main body having an anvil at an end of a substantiallyU-shaped frame and a female thread at the other end thereof; the spindlehaving a lead screw screwed with the female thread, being screwed withthe other end of the main body, and being advanced to and retracted fromthe anvil along with screwing rotation of the spindle; a detector thatdetects displacement of the spindle in the axial direction according toa rotation amount of the spindle; and a display unit that displays ameasurement value on the basis of a detection signal from the detector,in which a pitch of the lead screw may be twice as large or more thanthe difference between an external diameter and a core diameter of thelead screw, and the difference between the external diameter and thecore diameter may be one-fifth or less of the external diameter.

With this arrangement, since the spindle has the lead screw with thelarge pitch when an object to be measured is held between the anvil andthe spindle from the state that the anvil abuts on the spindle, the sameadvantages as the claim 1 can be obtained. In other words, the movementpitch per rotation of the spindle is increased, so that the spindle canbe moved at high speed, thus reducing effort and time spent on themeasurement.

Preferably, in the present invention, the detector may include: a statorprovided on the main body; a rotor facing to the stator; an engaginggroove provided on the spindle along the axial direction; an engagingpin provided on the rotor for engaging with the engaging groove; and apressurization unit that pressurizes the engaging pin toward theengaging groove.

With this arrangement, as the spindle is rotated, the rotation of thespindle is transmitted to the rotor because of the engagement betweenthe engaging groove of the spindle and the engaging pin of the rotor.Accordingly, the rotor is rotated only by the same rotating angle as thespindle, and besides, the rotating angle of the rotor is read by thestator. Thus, the rotating angle of the spindle can be found out, aswell as the displacement of the spindle by the pitch per rotation of thespindle.

According to the pressurization unit, since the engaging pin ispressurized toward the engaging groove, the engaging pin can securely befit to the engaging groove without clearance, thus transmitting therotation of the spindle to the rotor accurately. Thus, the reading errorof the rotating angle of the spindle by the detector can be reduced, andthe measurement accuracy can be enhanced.

Meanwhile, since the movement pitch per rotation of the spindle isenlarged by using the spindle that has the lead screw with the largepitch, the detection accuracy of the detector needs to be improved. Onlya slight clearance between the engaging pin and the engaging groove maycause a large effect on the measurement. However, the engaging pin canfit to the engaging groove without clearance due to the pressurizationunit, so that the engaging pin will not shake at the clearance relativeto the engaging groove, thereby realizing stable measurement.

Preferably, in the present invention, the engaging pin may be providedin a manner capable of sliding in a direction orthogonal to the axialdirection of the spindle, and the pressurization unit fixed on the rotorat an end thereof may have a leaf spring that pressurizes the engagingpin toward the engaging groove at the other end thereof.

With this arrangement, since bend elasticity of the leaf spring providespressurization for the engaging pin toward the engaging groove, theengaging pin can slide on the engaging groove as well as the engagingpin can fit to the engaging groove without clearance. Accordingly, therotation of the spindle can accurately be transmitted to the rotor.Owing to this, the reading error of the rotating angle of the spindle bythe detector can be reduced, and the measurement accuracy can beenhanced.

Preferably, in the present invention, the engaging groove may be formedin V-shape and a tip end of the engaging pin abutting on the engaginggroove may be formed in spherical shape.

With this arrangement, since the V-shaped groove is widened toward theupper side and narrowed toward the lower side, the tip end of theengaging pin is abutted on the both sides of the V-shaped engaginggroove without clearance. Besides, the tip end of the engaging pin isspherical, the contacting surface between the engaging pin and theengaging groove defines a point, the frictional force of which is small.Therefore, the engaging pin can slide on the engaging groove, as well asthe engaging pin can fit to the engaging groove without clearance.Consequently, the measurement accuracy of the measuring instrument canbe enhanced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an illustration showing the exterior of a micrometer accordingto an embodiment of the present invention;

FIG. 2 is a cross-sectional view of the micrometer of the aforesaidembodiment;

FIG. 3 is an illustration showing the profile of a lead screw of aspindle of the aforesaid embodiment;

FIG. 4A is an illustration showing a stator of the aforesaid embodiment;

FIG. 4B is an illustration showing a rotor of the aforesaid embodiment;

FIG. 5A is an illustration showing cross-sections of an engaging pin, anengaging groove and a pressurization unit of the aforesaid embodiment;

FIG. 5B is an illustration showing an enlarged primary portion of FIG.5A;

FIG. 5C is an illustration showing an enlarged primary portion of FIG.5A seen from different direction;

FIG. 6 is an illustration showing a modification of the fitting state ofthe lead screw with a female thread according to the present invention;

FIG. 7A is an illustration showing a modification of the statoraccording to the present invention; and

FIG. 7B is an illustration showing a modification of the rotor accordingto the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described below withreference to the attached drawings.

FIG. 1 shows a micrometer, which is a first embodiment of a measuringinstrument according to the present invention. FIG. 2 shows across-sectional view of FIG. 1.

This micrometer 1 includes a main body 2 having an anvil 223 at an endof a substantially U-shaped frame 222, a spindle 3 being screwed at theother end of the main body 2 and being advanced to and retracted fromthe anvil 223 in an axial direction along with its screwing rotation, adetector 4 that detects displacement of the spindle 3 in the axialdirection from a rotation amount of the spindle 3, and a digital display5 which is a display unit for displaying a measurement value on thebasis of a detection signal from the detector 4.

The main body 2 includes a front tube 22, a rear tube 21 and a spindlerotating portion 23 sequentially arranged from an end of the main body2.

The front tube 22 has a stem 221 provided on an opening of an end of thefront tube 22, and the U-shaped frame 222 provided on the outsidethereof. The U-shaped frame 222, which is provided with the anvil 223 atan end thereof to face to the spindle 3, is fixed to the front tube 22at the other end thereof, and besides, the digital display 5 is providedon the surface thereof.

The rear tube 21 is connected to the front tube 22 at an end thereof,has a female thread 211 at the inner periphery of the other end thereofto screw with the spindle 3, is formed with slitting 212 at the otherend thereof, and besides, is fixed by a nut 213 from the outsidethereof.

The spindle rotating portion 23 includes an inner guide tube 231 and anouter guide tube 232 sequentially layered on the rear tube 21, an outersleeve 233 rotatively provided on the outer guide tube 232, a thimble235 provided on the outer sleeve 233 with a friction spring 234 beinginterposed therebetween, and a cap tube 236 provided on the other endsof the outer sleeve 233 and the thimble 235. The cap tube 236 isconnected to the outer sleeve 233 by screwing with a screw.Additionally, a guide groove 237 is formed on the inner side of the captube 236 along the axial direction.

The spindle 3 projects from an end of the main body 2 to the outside bybeing inserted through the stem 221, and is provided with a lead screw31 on the outer periphery of the other end of the spindle 3 to screwwith the female thread 211 of the rear tube 21. A guide pin 32 isdisposed on the other end of the spindle 3 to engage with the guidegroove 237 of the cap tube 236. An engaging groove 40 is arranged on thespindle 3 along the axial direction.

As shown in FIG. 3, the lead screw 31 is a male screw which has arelatively large pitch P as well as a relatively small root thread depthd.

In other words, the pitch P of the lead screw 31 is large in which thepitch P is twice as large or more than the difference between itsexternal diameter R and its core diameter r, and the difference betweenthe external diameter R and the core diameter r is one-fifth or less ofthe external diameter R. When seeing along a screw axis line A, adjacentthread grooves 311 are formed with predetermined intervals, and anintermediate portion of thread grooves 312 is defined between theadjacent thread grooves 311 as a straight line along the screw axis lineA on a cross-section along the screw axis line A.

For instance, the lead screw 31 has, as its dimensions, approximately7.25 to 7.32 mm as the external diameter R, approximately 6.66 to 6.74mm as the core diameter r, approximately 1 to 2 mm as the thread pitchP, approximately 55 to 65 degrees as the apex angle θ of the screw rootthread, and approximately 5 degrees as the thread lead angle. Note that,the dimensions of the lead screw 31 are not limited particularly, andare appropriately chosen according to the lead, i.e., the advancementand retraction amount per rotation of the spindle 3. For example, thepitch P of the lead screw 31 may be triple, quintuple or decuple of thedifference between the external diameter R and the core diameter r, andthe difference between the external diameter R and the core diameter rmay be one-seventh or one-tenth of the external diameter R.

The female thread 211 has screw threads 214 on the lead screw 31 withthe same pitch. When seeing the female thread 211 along the screw axisline A, the adjacent screw threads 214 are formed with predeterminedintervals, and an intermediate portion of threads 215 is defined betweenthe adjacent screw threads 214 as a straight line along the screw axisline A on a cross-section along the screw axis line A.

The detector 4 has a stator 41 provided on the main body 2, a rotor 42facing to the stator 41, the engaging groove 40 formed on the spindle 3along the axial direction, an engaging pin 422 disposed on the rotor 42for engaging with the engaging groove 40, and a pressurization unit 6that pressurizes the engaging pin 422 toward the engaging groove 40.

The stator 41 is provided inside the front tube 22, and besides, isprovided on an end of the rear tube 21. A detent pin 411 is interposedbetween the stator 41 and the front tube 22 to restrict the stator 41from rotating. A spring 412 is interposed between the stator 41 and thefront tube 22 so that the stator 41 is biased toward an end thereof.

The rotor 42 has a rotor bushing 421, which can rotate independentlyagainst the spindle 3, the rotor 42 being arranged on the other end ofthe rotor bushing 421 to face to the stator 41.

The rotor bushing 421 is biased via a clumping collar 424 toward theother end by an adjustment screw 425 screwed with the stem 221.

The stator 41 and the rotor 42 constitute a rotary encoder ofelectromagnetic induction type.

As shown in FIG. 4A, the stator 41 includes a transmitting winding 419along the outer periphery of the stator 41 and a receiving winding 418disposed on the inner side of the transmitting winding 419. Thereceiving winding 418 has winding patterns 418A, 418B and 418C of threetypes each of which rhombuses are continuously formed, these patternsbeing arranged to be shifted to each other by one-third of the pitch ofthe rhombuses.

As shown in FIG. 4B, the rotor 42 has a flux couplement winding 429extending from the outer periphery to the inner periphery. The fluxcouplement winding 429 generates induction current due toelectromagnetic induction caused by the transmitting winding 419 of thestator 41, and the induction current is detected by the receivingwinding 418 of the stator 41. In the receiving winding 418, the signaloutput detected by each of the winding patterns 418A, 418B and 418Cvaries along with the relative rotation of the rotor 42 and the stator41. According to the variation, the angular strain of the rotor 42 andthe stator 41 can be detected.

In the present embodiment, corresponding to the male screw with thelarge lead of the spindle 3, a transmitting terminal 414 of the presentembodiment is constituted of 24 pieces, which is three times as many asa transmitting terminal 414 of related art having 8 pieces.

As shown in FIG. 5A, the engaging pin 422 is provided so as to sliderelative to the rotor bushing 421 in the axial direction, the engagingpin 422 engaging with the engaging groove 40 of the spindle 3.

As shown in FIG. 5B, the tip end of the engaging pin 422 is formed inspherical shape. The engaging groove 40 is formed in V-shape.

As shown in FIG. 5C, the pressurization unit 6, which is fixed on therotor bushing 421 at an end thereof, has at the other end thereof a leafspring 61 for pressurizing the engaging pin 422 toward the engaginggroove 40 as well as a fixing screw 62 for fixing an end of the leafspring 61 on the rotor bushing 421.

According to the micrometer 1 having the above-described configuration,when the cap tube 236 or the thimble 235 is rotated, the spindle 3 isrotated due to the engagement between the guide groove 237 and the guidepin 32. The spindle 3 is then advanced and retracted in the axialdirection by the spindle 3 screwing with the rear tube 21. When thespindle 3 is rotated, the rotor 42 is rotated by the engaging groove 40engaging with the engaging pin 422. When the rotating angle of the rotor42 is read by the stator 41, the rotating angle of the spindle 3 isdetected. The displacement of the spindle 3 is calculated according tothe rotating angle of the spindle 3 and the movement pitch per rotationof the spindle 3, and consequently, the displacement of the spindle 3 isdisplayed on the digital display 5 as the measurement value.

Thus, according to the micrometer 1 having the above-describedconfiguration, since the lead screw 31 of the spindle 3 has the largepitch, the displacement per rotation of the spindle 3 can be increased.Therefore, since the spindle 3 can realize high-speed movement, themicrometer 1 can provide high operational performance.

Since the leaf spring 61 is provided and the engaging pin 422 ispressurized to the engaging groove 40 by the leaf spring 61, theengaging pin 422 can be abutted on the engaging groove 40 withoutclearance. Additionally, since the tip end of the engaging pin 422 isformed in spherical shape, and the engaging groove 40 is formed inV-shape, the spherical tip is abutted on the both sides of the V-shape,so that the engaging pin 422 can slide on the engaging groove 40 as wellas the engaging pin 422 can be abutted on the engaging groove 40 withoutclearance. Accordingly, since the engaging pin 422 will not shake at theclearance relative to the engaging groove 40, measurement accuracy ofthe micrometer 1 can be enhanced.

Since the transmitting terminal 414 of the stator 41 contains 24 pieces,the number of which is three times as great as that of related art, therotating angle of the spindle 3 can highly accurately be detected. Sincethe lead screw 31 of the spindle 3 has the large pitch, the movementpitch per rotation of the spindle 3 is increased. In the presentembodiment, since detection accuracy of the rotor 42 and the stator 41is enhanced, the measurement accuracy will not be deteriorated even whenthe lead screw 31 of the spindle 3 has the large pitch.

Note that, the measuring instrument of the present invention is notlimited to the above-described embodiment, and it is obvious thatvarious modifications can be applied thereto as long as the object ofthe present invention can be attained.

While the female thread 211 has the intermediate portion of threads 215defined between the adjacent screw threads as a straight line along thescrew axis line A on a cross-section along the screw axis line A, forinstance, the female thread 211 may be formed by a large screw thread214 as shown in FIG. 6. The machining allowance of the screw thread 214will be increased as the scale of the screw thread 214 is enlarged,however, the strength thereof can be maintained if the rear tube 21 issufficiently thick even when the machining allowance of the screw thread214 is increased. In the case that the female thread with the largepitch is machined, the height of the screw thread 214 should beincreased, so that the machining of the female thread will be simplifiedand thus the machining cost thereof can be decreased.

The pressurization unit 6 is not limited to the leaf spring 61, and anycomponent can be applied instead as long as the pressurization unit 6can provide pressurization such as oil hydraulics.

While the rotary encoder of electromagnetic induction type is employedas the detector in the above-described embodiment, various sensors suchas a rotary encoder of electrostatic capacitance type or photoelectrictype can be utilized.

For example, the stator 41 is provided with a receiving terminal 413 andthe transmitting terminal 414 as shown in FIG. 7A and the rotor 42 isprovided with a coupled terminal 423, so that the angular strain isdetected on the basis of the variation on the electrostatic capacitanceof these terminals.

The measuring instrument is not limited to the micrometer, and anymeasuring instrument can be applied thereto such as a micrometer head aslong as the spindle can be advanced and retracted along with itsrotation.

INDUSTRIAL APPLICABILITY

The present invention can be used as a measuring instrument thatmeasures dimensions etc. of a workpiece by advancing and retracting aspindle along with screwing rotation thereof, for instance, typically asa micrometer or a micrometer head.

1. A measuring instrument, comprising: a measuring means, the measuringmeans including: a main body having a female thread; and a spindlehaving a lead screw screwed with the female thread and capable of beingadvanced and retracted in an axial direction along with rotation aroundthe axial center, wherein a pitch of the lead screw is twice as large ormore than the difference between an external diameter and a corediameter of the lead screw, and the difference between the externaldiameter and the core diameter is one-fifth or less of the externaldiameter, wherein the measuring means determines movement of the spindlerelative to the main body.
 2. The measuring instrument according toclaim 1, wherein the main body has an anvil at an end of a substantiallyU-shaped frame and a female thread at the other end thereof, the spindlehas a lead screw screwed with the female thread, is screwed with theother end of the main body, and is advanced and retracted against theanvil along with screwing rotation of the spindle, the measuringinstrument, including: a detector that detects displacement of thespindle in the axial direction according to a rotation amount of thespindle; and a display unit that displays a measurement value on thebasis of a detection signal from the detector.
 3. The measuringinstrument according to claim 2, the detector, including: a statorprovided on the main body; a rotor facing to the stator; an engaginggroove provided on the spindle along the axial direction; an engagingpin provided on the rotor for engaging with the engaging groove; and apressurization unit that pressurizes the engaging pin toward theengaging groove.
 4. The measuring instrument according to claim 3,wherein the stator and the rotor constitute a rotary detector ofelectromagnetic induction type.
 5. The measuring instrument according toclaim 3, wherein the stator and the rotor constitute a rotary detectorof electrostatic capacitance type.
 6. The measuring instrument accordingto claim 3, wherein the engaging pin is provided in a manner capable ofsliding in a direction orthogonal to the axial direction of the spindle,the pressurization unit fixed on the rotor at an end thereof has a leafspring at the other end thereof to pressurize the engaging pin towardthe engaging groove.
 7. The measuring instrument according to claim 3,wherein the engaging groove is formed in V-shape and a tip end of theengaging pin abutting on the engaging groove is formed in sphericalshape.
 8. The measuring instrument according to claim 1, whereinadjacent thread grooves of the lead screw are formed to havepredetermined intervals in a direction along a screw axis line, and anintermediate portion of thread grooves is defined between the adjacentthread grooves as a straight line along the screw axis line on across-section along the screw axis line.
 9. The measuring instrumentaccording to claim 1, wherein the female thread has screw threads of thesame pitch as that of the thread grooves, as seen in a direction alongthe screw axis line of the female thread, the adjacent screw threadsbeing formed with predetermined intervals, and an intermediate portionof threads being defined between the adjacent screw threads as astraight line along the screw axis line on a cross-section along thescrew axis line.